Shoulder Anatomy and Biomechanics

Key Points

The shoulder complex is composed of three bones—the clavicle, the scapula, and the humerus—as well as four articulations—the acromioclavicular (AC), the sternoclavicular (SC), the scapulothoracic, and the glenohumeral (GH) joints.

The clavicle serves a variety of functions. It acts as a rigid base for muscular attachments of the shoulder, neck, and chest. It also provides protection for the major vessels at the base of the neck and for the nerves and vessels supplying the upper limb. In addition, it forms a strut that holds the GH joint in the parasagittal plane.

The scapula is a flat, triangular-shaped bone that serves as the articulating surface for the head of the humerus as well as provides areas for 17 muscle attachments.

The proximal humerus is composed of the humeral head, the lesser and greater tuberosities, the bicipital groove, and the proximal humeral shaft.

The AC joint is a diarthrodial joint and the only articulation between the clavicle and the scapula. The motion of the AC joint is minimal, involving small translations and, mainly, rotation between the clavicle and the acromion with arm movement.

The SC joint is the only true joint that connects the upper extremity to the axial skeleton. It is a gliding joint with little inherent bony stability.

The scapulothoracic articulation is not a true joint, but it represents the space between the concave surface of the anterior scapula and the convex surface of the posterior chest wall. The muscular and ligamentous attachments provide the stability of this articulation as the scapula retracts, protracts, and rotates along the posterior chest wall.

The GH joint is a diarthrodial joint with minimal bony constraint, allowing it the largest range of motion of any major diarthrodial joint in the body.

The glenoid labrum provides another static restraint to GH motion. The labrum is a fibrous ring that is attached to the glenoid articular surface through a fibrocartilagenous transition zone.

The GH ligaments function as static restraints to shoulder motion.

The rotator cuff (RTC) muscles, as well as the scapular rotators, contribute to GH stability by enhancing the concavity–compression mechanism. Contraction of the long head of the biceps tendon, coordinated scapulothoracic rhythm, and proprioceptive mechanoreceptors in the joint capsule also contribute to the stability.

The RTC is composed of the supraspinatus, the infraspinatus, the subscapularis, and the teres minor muscles. The RTC often serves more than one function simultaneously. The muscles act as prime movers if the line of action is within the intended direction of motion.

Nerve injuries can occur with both arthroscopic and open shoulder procedures.

RTC repair using an open technique has been clinically successful in terms of repair, although arthroscopic techniques are now more common.

The shoulder is a complex joint that has the greatest degree of mobility of all major joints in the human body. The osseous and ligamentous structures that comprise the joint, as well as the surrounding musculature, interact to provide a wide range of motion as well as stability. Under normal conditions, four articulations move in synchrony, allowing smooth, unhindered motion of the arm.

The control of glenohumeral (GH) stability is achieved by the complex interaction between the static restraints (i.e., the ligament and tendons) and the dynamic restraints (i.e., muscular contraction acting across the joint).